Image processing method and apparatus with reduction of granular noise, etc.

ABSTRACT

According to the present invention, when image data are subjected to quantization processing using an error diffusion method, the generation of a striped pattern which is a cause for deterioration of picture quality in the error diffusion method is prevented. In the present invention, an original image is read and image data are generated. The image data obtained by reading the original image are quantized using an error diffusion method, and the quantized image data can be output and recorded by a thermal-head printer. Furthermore, according to the present invention, an error between the density of an input image and the density of an output image after being subjected to a quantization processing using an error diffusion method is perfectly preserved. In an image processing apparatus for performing a quantization by dispersing an error between input image data and output image data which arises when the input image data are quantized, to image data of surrounding picture elements, the error between the input image data and the output image data is computed or otherwise determined, the error is subjected to a predetermined weighting processing, the error subjected to the weighting processing is dispersed to surrounding picture elements, and a surplus of the error generated in the weighting processing is corrected.

This application is a continuation of application Ser. No. 07/346,906filed May 3, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image processing method and apparatus forperforming a quantization processing of image data, and moreparticularly, to an image processing method and apparatus forreproducing a half-tone image.

2. Description of the Prior Art

Heretofore, there has been known an error diffusion method for use as animage processing method for reproducing a half-tone image by, forexample, a binary image reproducing, in a digital copier, digitalfacsimile and the like.

In this method, a difference in density, for every picture element,between the density of an image of an original document and the densityof the corresponding picture element of the binary-coded output image,that is, an error, is determined, and the value of the error sodetermined is dispersed, after performing a specific weighting topicture elements surrounding the picture element in question inaccordance with coefficients of a weighting matrix.

Since this method spatially clears an error which is a difference indensity between an image of an original document and an output image,there is no limitation on the number of gradations due to the size of amatrix as in a dither processing (which is another binary-codingmethod), and it is possible to perform a threshold processing whichdepends on the value of a picture element.

Accordingly, the error diffusion method makes possible compatibility ofgradation and resolution, which is a problem in dither processing.

The error diffusion method has been presented in R. W. Floyd and L.Steinberg, "An Adaptive Algorithm for Spatial Gray Scale", SID75 Digest(1976).

The expression of the error diffusion method is as follows. In thefollowing expression, input data are assumed to be of 6 bits;

    D.sub.i,j =X.sub.i,j +(ΣΣα.sub.i+m,j+n.E.sub.i+m,j+n)(1/Zα.sub.m,n)

    Y.sub.i,j =63(D.sub.i,j ≧T)

    Y.sub.i,j =0(D.sub.i,j <T),

where

D_(i),j : the density of the picture element (i,j) in question aftercorrection

X_(i),j : the density of an input image of the picture element (i,j) inquestion

E_(i),j : the error when the picture element (i,j) in question isbinary-coded

α_(i),j : weighting cofficient

Y_(i),j : the density of an output image

T: threshold value.

That is, in the above expression, weighted (by multiplying α_(i+m),j+nand dividing by Σα_(m),n) values of errors E_(i+m),j+n generated atsurrounding picture elements are added to the density X_(i),j of aninput image of the picture element in question, and the resultant valuebecomes the density D_(i),j of the picture element in question aftererror correction. The density Y_(i),j of an output image is obtained bybinary-coding the D_(i),j using a threshold value T (for example, T=32).

A printer performs an on/off control of a dot (i.e., prints a dot ornot) in accordance with the value of the Y_(i),j to perform imageformation.

However, when highlight portions of an image are binary-coded by theerror diffusion method, there is the disadvantage that grain-like noisesare generated in the highlight portions of the image. In order to removethis disadvantage, the assignee of the present invention has filed Ser.No. 289,017.

The error diffusion method also has the disadvantage that a uniquetexture (a striped pattern) appears in high-light and half-tone portionsof an image. This is caused by dots of binary outputs connected in aline.

Now, the cause of such generation of the texture will be investigated.As described above, in the error diffusion method, an error generated ina picture element in question is weighted using a weighting matrix anddiffused to surrounding picture elements.

For example, a weighting matrix α_(i),j (X,1), that is, for a case inwhich an error generated at a picture element X in question is dispersedto an adjacent picture element to the right, will be investigated.

Since there is a higher probability of an output image being 0 inhighlight and half-tone portions of an image compared with darkportions, a positive error is generated in many cases. A positive erroris generated when an output image is made 0, since input image data haveat least a certain degree of density.

When the positive error is dispersed to an adjacent picture element tothe right with the above-described weighing matrix α_(i),j (X,1), theprobability of a dot being "on" at the dispersed picture element (theadjacent picture element) becomes high. When processing for one line ofinput image data is completed and the processing is shifted to the nextline, the positive error is also dispersed to a picture element whichcorresponds to that in the preceding line (a picture element under thatin the preceding line), and the probability of the dot of this pictureelement being "on" also becomes high.

That is, the probability of dots being "on" becomes high periodically inthe subscanning direction, and a striped pattern is generated due toconnection of these dots. An appearance of the generation of a stripedpattern in the subscanning direction is shown in FIG. 25.

According to the shape of the weighting matrix, dots may also beconnected in the main scanning direction or in an oblique direction, anda striped pattern is generated.

As described above, although, in the conventional error diffusionmethod, resolution is excellent compared with dither processing, aunique texture (this striped pattern) is generated in highlight andhalf-tone portions of an image, and it is impossible to reproduce anexcellent image.

Now, in the error diffusion method, the processing for determiningvalues to be distributed to surrounding picture elements from the errorgenerated at a picture element in question will be investigated.

The error generated when the density X_(i),j of an input image of apicture element (i,j) in question is binary-coded is represented byE_(i),j, and the weighting matrix α_(i),j is represented by ##EQU1## X:a picture element in question.

In order to determine distribution values, first the error E_(i),j isdivided by the sum 10 of numbers which make up the weighting matrixα_(i),j, and values in which each coefficient of α_(i),j is multipliedby that sum become distribution values of the E_(i),j to surroundingpicture elements.

For example, if E_(i),j =25, the values become

    ______________________________________                                        to picture element (i + 1, j)                                                                      4*Int(25*1/10) = 8                                       to picture element (i - 1, j)                                                                      1*Int(25*1/10) = 2                                       to picture element (i, j + 1)                                                                      4*Int(25*1/10) = 8                                       to picture element (i + 1, j + 1)                                                                  1*Int(25*1/10) = 2.                                      ______________________________________                                    

In this example, the configuration is provided by hardware, and isdesigned to truncate values to the right of the decimal point for thesake of simplification.

When the above-calculated distribution values are added, the result is

    E*.sub.i,j =8+2+8+2=20.

This value is different from E_(i),j =25.

The difference (E_(i),j -E*_(i),j) is caused by neglecting the remainderwhen the error is divided by 10.

In the case of the error diffusion method, if there is a differencebetween the error generated at a picture element in question and theerror diffused to surroundings, the density of an input image is notpreserved. Hence, it results that the density of an input image does notequal the density of an output image, and the picture quality of theoutout image deteriorates.

When a decimal-point operation (it is necessary to execute adecimal-point operation of at least two digits in order to prevent thedeterioration of an image) is used in order to solve the above-describedproblems, circuit scale becomes very large, and so this approach is notan effective method.

As described above, the conventional error diffusion method has thedisadvantage that, if an error due to a remainder or surplus which isgenerated when an error is weighed is neglected, density is notpreserved, and picture quality deteriorates.

There is also the disadvantage that, if it is tried to suppress theinfluence of the remainder by performing a decimal-point operation,circuit scale becomes very large.

SUMMARY OF THE INVENTION

The present invention removes the above-described disadvantages of theprior art.

It is an object of the present invention to provide an image processingmethod and apparatus which can excellently reproduce all kinds of inputimages.

It is a further object of the present invention to provide an imageprocessing method and apparatus which can suppress the generation of astripped pattern which is a problem in an error diffusion method, byquantizing input image data using an error diffusion method andoutputting and recording the quantized image data by means of athermal-head printer.

It is a still further object of the present invention to provide animage processing method in which one apparatus transmits binary datawhich have been binary-coded by an error diffusion method to anotherapparatus, and the latter apparatus performs recording by means of athermal-head printer according to the binary data sent from the oneapparatus.

It is still another object of the present invention to provide an imageprocessing method and apparatus which can reproduce an image which isfaithful to an original image by recording image data subjected toquantization processing by an error diffusion method varying the sizesof dots.

It is still a further object of the present invention to provide animage processing method and apparatus which can preserve the density ofan input image and the density of an output image with a device of asimple configuration, when image data are quantized by means of an errordiffusion method.

These and other objects are accomplished, according to the preferredembodiments, by an image processing apparatus for performingquantization by dispersing an error between input image data and outputimage data when the input image data are quantized to image data ofsurrounding picture elements, the apparatus comprising:

arithmetic means for determining an error between the input image dataand output image data;

processing means for performing a predetermined weighting processing tothe error;

means for dispersing the error subjected to the weighting processing bythe processing means; and

correction means for correcting a remainder of the error which isgenerated when the weighting processing is performed in the processingmeans.

Still another object of the invention is to provide image processingmethod and apparatus which can eliminate particle-like noises in ahighlight portion which are generated by the binarization process in anerror diffusion method and can also improve an encoding efficiency.

These and other objects, features and advantages of the presentinvention will become more fully apparent from the following detaileddescription of the preferred embodiments taken in connection with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a first embodiment of the presentinvention;

FIG. 2 is a block diagram showing the details of a reader unit 1 in FIG.1;

FIG. 3 is a diagram showing a brightness-density conversion table;

FIG. 4, which is divided into FIGS. 4A and 4B, is a block diagramshowing the details of a binary-coding processing unit;

FIG. 5 is a diagram showing an example of a weighing matrix;

FIG. 6, consisting of FIGS. 6(a), 6(b) and 6(c) is a diagram showingflows of binary-coding processings in an error diffusion method;

FIG. 7 is a diagram showing a relationship between driving current andlight power of a semiconductor laser 110;

FIG. 8 is a diagram showing energy distributions of a laser light;

FIG. 9 is a diagram showing the sizes of dots which are determined bybinary data and density data;

FIG. 10 consisting of FIGS. 10(a), 10(b) and 10(c), is a diagram showingexamples of printing in the first embodiment;

FIG. 11 is a block diagram showing second embodiment of the presentinvention;

FIG. 12 is a block diagram showing the detail of a thermal-head printer;

FIG. 13 consisting of FIGS. 13(a), 13(b) and 13(d), is a diagram showingexamples of printing in the second embodiment;

FIG. 14, which is divided into FIGS. 14A and 14B, is a block diagramshowing another example of a binary-coding processing unit;

FIG. 15 is a diagram showing a weighing mask used in the binary-codingprocessing unit in FIG. 14;

FIG. 16 is a diagram showing positions of the error distribution whichis performed in the binary-coding processing unit in FIG. 14;

FIG. 17, which is divided into FIGS. 17A and 17B, is a block diagramshowing a third embodiment of the present invention;

FIG. 18, which is divided into FIGS. 18A and 18B, is a block diagramshowing the detail of a binary-coding processing unit in the thirdembodiment;

FIGS. 19, 22 and 23 are recording-control flow charts;

FIGS. 20, 21 and 24 are diagrams showing examples of recording in thethird embodiment; and

FIG. 25 is a diagram showing a problem in the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be hereinafterexplained with reference to the drawings.

First Embodiment

As a first embodiment, a case in which a laser-beam printer (LBP) isused in a recording unit will be explained.

FIG. 1 is a block diagram showing the first embodiment of the presentinvention. In FIG. 1, a reader unit 1 comprises an image input unit 101for reading an image of an original document, and an image processingunit 102 in which input image data are subjected to a quantizationprocessing into binary data by an error diffusion method. A randomaccess memory (RAM) 103 is used for image processing and as workingareas for a CPU 104. The CPU 104 controls the reader unit 1 and an LBPrecording unit which will be described later.

Control signals and image data in each block are transferred through asystem bus 105. A read-only memory (ROM) 106 stores control programswhich the CPU 104 executes. An LBP control circuit 107 controls the LBPby the control of the CPU 104.

An LBP recording unit 2 performs image formation according to data whichare binary coded by the error diffusion method. The LBP recording unit 2comprises a laser control circuit 108 for controlling the light power ofa laser beam by controlling driving current, a laser driving circuit 109for driving a laser by means of the driving current which is controlledby the laser control circuit 108, a laser light source 110 consisting ofa semiconductor laser, a rotating polygonal mirror 111 for performingdeflection scanning of the light beam on the surface of a photosensitivebody 115, an imaging lens 112 for imaging the light beam on the surfaceof the photosensitive body 115, a driving unit 113 for driving therotating polygonal mirror 111, and a corona charger 114, thephotosensitive body 115, recording paper 116, a charger 117 for transferand a developer 118 for visualizing a latent image formed on thephotosensitive body 115 by laser beam 3. It is possible to control thesize (diameter) of a resulting dot by means of controlling the drivingcurrent by the laser control circuit 108.

A communication control unit 4 performs control when the imageprocessing unit 102 communicates binary-coded data by the errordiffusion method, and comprises an image memory, a coding/decoding unitfor coding and decoding data or a modulating/demodulating unit of data,and the like. The communication control unit 4 is controlled by acommunication program stored in the ROM 106.

A receiver 5 is connected to the communication control unit 4 via acommunication network. The receiver 5 has an LBP recording unit as theLBP in the present embodiment has. An operation unit 6 is used forentering such information as the identity (name and/or telephone number)of communication correspondents and the like.

FIG. 2 is a block diagram showing the detail of a reader unit 1 in FIG.1.

An input sensor unit 201 comprises a photoelectric converter, such as aCCD or the like, and driving device for scanning the photoelectricconverter, and performs read scanning of an original document.

An A/D converter 202 converts image data read by the input sensor unit201 into digital signals having a quantization number of 6 bits. Thenumber of gradations is thus 64 steps. Data 000000, for the lowestbrightness, indicates the most dense black, and data 111111, for thehighest brightness, indicates white.

Brightness data from the A/D converter 202 are sent to a correctioncircuit 203, where the correction of unevenness in sensitivity of theCCD and of shading distortion which is a distortion, in thelight-distribution characteristic of the light source, is performed.

A conversion table 204 converts brightness data from the correctioncircuit 203 into density data, and comprises a ROM which outputs,corresponding to 6 bits of input brightness data, 6 bits of densitydata. In general, there is the relationship (density)=-γ log(brightness) (γ: a positive constant) between brightness and density.Data according to this formula are written in the table 204. An exampleof the content of the conversion table is shown in FIG. 3.

In the correspondence table in FIG. 3, the input data values (brightnessdata) of 60 or more (white) are all set such that the output data values(density data) are set to 0. The output data X_(i),j is the density data[0 (white)-63 (black)].

That is, with respect to the brightness data of 60 or more, in the caseof performing the binarization using the error diffusion method byconverting the density data to 0, an increase amount of the error datawhich is distributed to the peripheral pixels is also 0 in the portionhaving density data of 0. Therefore, by adding the error data, the valueof the density data of certain pixels exceeds the threshold value, sothat output of such spurious black dots can be prevented.

As described above, according to this embodiment, the brightness densityconversion table is set before the binarizing process based on the errordiffusion method and the brightness data of a predetermined value ormore is set to the density data 0, so that the increase amount of theerrors in the error diffusion method can be set to 0. The particle-likenoise in the highlight portion can be reliably eliminated at a highspeed.

On the other hand, by eliminating the particle-like noise in thehighlight portion, the encoding process in the facsimile apparatus canbe also efficiently executed.

A binary-coding processing unit 205 performs quantization processing of6-bit density data sent from the conversion table 204 into a 1-bitbinary signal by the error diffusion method.

FIG. 4 is a block diagram showing the detail of the binary-codingprocessing unit 205 in FIG. 2.

Data X_(i),j sent from the conversion table 204 is added by an adder401, to error data E_(i),j from an adder 406 which have already beengenerated in the course of performing the binary coding processing. Thedata D_(i),j corrected by the error is represented by the followingexpression;

    D.sub.i,j =X.sub.i,j +E.sub.i,j.

The data D_(i),j is binary-coded using the threshold T (T=32) in acomparator 402. That is, the binary-coded output Y_(i),j is representedas follows;

    D.sub.i,j ≧T--Y.sub.i,j =63

    D.sub.i,j <T--Y.sub.i,j =0.

The data D_(i),j is also sent to an error arithmetic unit 403. In theerror arithmetic unit 403, an error E_(i),j to be distributed tosurrounding picture elements is calculated according to the data D_(i),jand the binary-coded output Y_(i),j. That is, the E_(i),j can berepresented as follows;

    E.sub.i,j =D.sub.i,j -Y.sub.i,j.

The data E_(i),j is sent to an error-distribution-value arithmeticcircuit 404, where values to be distributed to four picture elementssurrounding a picture element in question are determined.

FIG. 5 is a diagram showing a weighting matrix, which indicates thepositions and ratios of picture elements to which the error E_(i),jgenerated at the picture element X in question is distributed.

In the error-distribution-value arithmetic circuit 404, A_(i),j andB_(i),j in FIG. 5 are determined as follows;

    A.sub.i,j =2×Int(E.sub.i,j ×1/6)

    B.sub.i,j =Int(E.sub.i,j ×1/6)

The error-distribution-value arithmetic circuit 404 is configured totruncate values to the right of the decimal point. That is, it ispossible to execute only operations upon integers. "Int" represents thatvalues to the right of the decimal point are truncated. By truncatingvalues to the right of the decimal point, there is generated a surplusor difference R_(i),j between the error E_(i),j generated at the pictureelement in question and the A_(i),j and B_(i),j which have been computedat the error-distribution-value arithmetic circuit 404 and which are tobe dispersed to the surrounding four picture elements. The differenceR_(i),j is represented by the following expression;

    R.sub.i,j =E.sub.i,j -2×(A.sub.i,j +B.sub.i,j).

The difference R_(i),j is sent to a latch 405 to be delayed by onepicture element, and added to input data X_(i+1),j of the next pictureelement.

For example, if it is assumed that the density (X_(i),j) of the originalimage of the picture element in question is (34), and the sum (E_(i),j)of errors diffused from surrounding picture elements to the pictureelement in question is (-9), D_(i),j becomes 25. If the threshold valueis assumed to be (32), the density of an output image becomes 0, and theerror E_(i),j becomes E_(i),j =25. Error values to be distributed tosurrounding picture elements which are determined from E_(i),j =25according to the weighting matrix in FIG. 5 are calculated, for P whichis picture element (i+1, j), as, ##EQU2## for Q which is picture element(i-1, j+1), as, ##EQU3##

Errors to be distributed to other picture elements can be described asfollows; ##EQU4## When the values of P, Q, R and S calculated above areadded, ##EQU5## This value is different from E_(i),j =25, and the errorvalue becomes smaller by 1.

In the case of the error diffusion method, if there is a differencebetween an error generated in a picture element in question and an errorto be diffused to surrounding picture elements the density of the inputimage is not equal to the density of the output image, and the picturequality of the output image deteriorates. Hence, in the presentembodiment, a surplus of the error generated as a result of operation,that is, 1 in the above-described example, is not truncated, and thesurplus 1 is carried over when the picture element in question isshifted from (i,j) to (i+1,j).

The surplus is R_(i),j in FIG. 4.

On the other hand, the value A_(i),j is sent to adders 413 and 408 inorder to be distributed to picture elements (i+1,j) and (i,j+1),respectively, and the value B_(i),j is sent to a latch 407 and an adder410 in order to be distributed to picture elements (i+1,j+1) and(i-1,j+1), respectively.

A memory 411 is a memory for storing errors to be distributed to thej+1-th line, and can store error data of picture elements of at leastone line.

A timing generation circuit 415 generates various kinds of signals, suchas latch signals for latch circuits 405, 407, 409, 412 and 414, addresssignals for the memory 411 and the like.

Next, the method of distributing the above-described errors will beexplained in more detail with reference to FIG. 6.

FIG. 6 is a diagram showing flows of binary-coding processing by theerror diffusion method. First, if values weighted to an error generatedat the picture element X₁ in question are represented by P₁, Q₁, R₁ andS₁, these values are dispersed to four surrounding picture elements asshown in FIG. 6(a). The P₁, Q₁ and R₁ and S₁ are sent to adders 413, 410and 408, and a latch 407 in FIG. 4, respectively. The value Q₁ iswritten in address 1 of the memory 411.

Next, when the picture element in question is shifted to X₂, errors P₂,Q₂, R₂ and S₂ are dispersed to four surrounding picture elements asshown in FIG. 6(b). The value P₂ is sent to the adder 413. The value Q₂is added to the R₁ generated at the X₁ in the adder 410, and written inaddress 2 of the memory 411. The value R₂ is added to S₁ generated at X₁in the adder 408. The value S₂ is sent to the latch 407.

Next, when the picture element in question is shifted to X₃, errors P₃,Q₃, R₃ and S₃ are dispersed to four surrounding picture elements asshown in FIG. 6(c). The value P₃ is sent to the adder 413. The value Q₃is added to S₁ generated at X₁ and R₂ generated at X₂ in the adder 410,and written in address 3 of the memory 411. R₃ is added to S₂ generatedat X₂ in the adder 408. S₃ is sent to the latch 407.

When the above-described processing is performed for one line, thefollowing values are written in the memory 411:

    ______________________________________                                        address 1 in the memory                                                                          M.sub.1 = Q.sub.1                                          address 2 in the memory                                                                          M.sub.2 = R.sub.1 + Q.sub.2                                address 3 in the memory                                                                          M.sub.3 = S.sub.1 + R.sub.2 + Q.sub.3                      address 4 in the memory                                                                          M.sub.4 = S.sub.2 + R.sub.3 + Q.sub.4                      address i in the memory                                                                          M.sub.i = S.sub.i-2 + R.sub.i-1 + Q.sub.i                  ______________________________________                                    

When the processing for one line has been completed and the processingproceeds to the next line, errors generated at the preceding line areread from the memory.

The errors read from the memory are added to an error generated at thepreceding picture element in the adder 413 and output from the latch414.

The reading of errors from the memory 411 is controlled by the timinggeneration circuit 415 so as to correspond to the preceding line. Thetiming generation circuit 415 controls so that, if the picture elementin question is X_(i), address M_(i-3) in the memory 411 is read.

By means of performing the above-described processing for all inputdata, it is possible to perform a binary coding by the error diffusionmethod.

As explained above, according to the present embodiment, it isconfigured such that a surplus of an error generated when the error isdispersed with weighing in the error diffusion method is added to inputimage data of the next picture element. Hence, it is possible to preventthe deterioration of picture quality with a simple configuration,without performing a decimal point operation, which would require alarge-scale hardware.

A comparator 416 in FIG. 4 determines to which among highlight signal,dark signal and half-tone signal the input picture signal X_(i),jbelongs, and outputs a flag for respective signal.

That is, the comparator 416 compares X_(i),j with two threshold valuesTD₁ and TD₂ (TD₁ <TD₂),

    ______________________________________                                        X.sub.i,j ≦ TD.sub.1                                                                   ∴Flag = 0 (highlight signal)                          TD.sub.1 > X.sub.i,j > TD.sub.2                                                               ∴Flag = 1 (half-tone signal)                          X.sub.i,j ≧ TD.sub.2                                                                   ∴Flag = 2 (dark signal)                               ______________________________________                                    

and outputs a flag in accordance with each gradation level.

Next, the recording processing when image data binary-coded in thereader unit 1 in FIG. 1 are recorded at the LBP recording unit 2 will beexplained.

By the control of the CPU 109, the LBP control circuit 107 transfersbinary image data Data from the image processing unit 102, a densityflag Flag (417) and a clock signal Ck to the laser control circuit 108.

In the present embodiment, the light-beam diameter is changed inaccordance with each content of Flag. As means for this purpose, amethod for changing driving current is used.

FIG. 7 shows a relationship between drive current and light power of asemiconductor laser 10.

In the present embodiment, in the case of Flag=0 (a high-contrastpicture-element density), the driving current is I₁ and the light poweris LP₁.

In the case of Flag=2 (a dark picture-element density), the drivingcurrent is I₂ and the light power is LP₂.

In the case of Flag=1 (a half-tone picture-element density), the drivingcurrent is randomly selected between I₁ and I₂.

Since the laser light has a Gaussian energy density distribution, thelight-power density distribution becomes D₂ when the driving current isincreased from that for the light-power density distribution D₁. Whenthe amount of exposure necessary for recording on the photosensitivebody is represented by E_(r), the diameter of a recording dot on thephotosensitive body is varied according to the driving current.

The diameter of a recording dot r is expressed by ##EQU6## where P_(o)is the output of the laser light, and a and b are constants.

The driving current is converted into a light beam by the laser lightsource 110, and the emitted light beam performs deflection scanning onthe surface of the photosensitive body 115 by the rotating polygonalmirror 111. The imaging lens 112 images the light beam on the surface ofthe photosensitive body 115.

After the photosensitive body 115 has been charged by the corona charger114, the light beam 3 projects an image on the photosensitive body 115to form an electrostatic latent image.

The electrostatic latent image is visualized by the developer 118, andtransferred to the recording paper 116 by the transfer charger 117.

FIG. 9 is a diagram showing the sizes of printed dots which arecontrolled in accordance with data 801 which have been binary coded bythe error diffusion method and the content 802 of Flag which indicatesthe density of an input image. That is, when the value of thebinary-doded data 801 is 1 (dot "on") and the content of the Flag 802 is2 (a dark portion), the size of a printed dot becomes large, and whenthe value of the binary-coded data 801 is 1 and the content of the Flag802 is 0 (a highlight portion), the size of a printed dot becomes small.

When the value of the binary-coded data 801 is 1 and the content of theFlag 802 is 1 (half-tone portion), the size of a printed dot is switchedbetween large and small.

FIG. 10 shows a recording example using the dot sizes shown in FIG. 9.

FIG. 10(a) shows a highlight portion of an output image, in which aconnection between dots is prevented because the dot size is small. Thatis, the generation of a unique striped pattern in the error diffusionmethod can thereby be prevented.

FIG. 10(b) shows a half-tone portion of the output image. In this case,a connection between dots can be prevented by printing small dots, andthus it is possible to prevent the generation of the mentioned uniquestriped pattern in the error diffusion method. In this case, moreover,since two kinds, that is, large and small, of dot sizes are printed withswitching, it is possible to provide gradation in the half-tone portion.

FIG. 10(c) shows a dark portion of the input image. In this case, sincelarge dots are printed, it is possible to prevent the generation ofwhite spaces between dots.

As described above, according to the first embodiment of the presentinvention, by changing the size of a printed dot in accordance with thedensity of an input image, it is possible to prevent the generation of aunique striped pattern in the error diffusion method in highlight andhalf-tone portions of an image, and further to prevent the generation ofwhite spaces in dark portions of the image.

Further, according to the first embodiment, the brightness densityconversion table is set before the binarizing process based on the errordiffusion method and the brightness data of a predetermined value ormore is set to the density data 0, so that the increase amount of theerrors in the error diffusion method can be set to 0. The particle-likenoises in the highlight portion can be certainly eliminated at a highspeed.

On the other hand, by eliminating the particle-like noises in thehighlight portion, the encoding process in the facsimile apparatus canbe also efficiently executed.

Furthermore, as shown in FIG. 1, it is possible to send binary data anda flag indicating the density level of an image to the receiver 5 viathe communication control unit 4.

When data are sent to an apparatus in communication, the CPU 104performs control in accordance with transmission instruction from theoperating unit 6.

The receiver 5 is provided with a recording unit having the sameconfiguration as that of the LBP recording unit 2. This recording unitperforms recording controlling the size of the diameter of a dotaccording to binary data and a flag which have been sent.

Although, in the first embodiment, the density of a picture element inquestion is divided into three density levels and the diameter of aprinted dot is changed in accordance with each density level, a stripedpattern which is unique in the error diffusion method can also bereduced by the following methods.

(i) The diameter of a dot is randomly changed irrespective of thedensity level.

(ii) The diameter of a dot which is smaller than the standard diameterof a dot is used for a picture element having a high-contrast density,and the density of a dot is randomly changed for other density levels.

(iii) The standard diameter of a dot is used for a picture elementhaving a dark density, and the diameter of a dot is randomly changed forother density levels.

(iv) The diameter of a dot which is smaller than the standard diameterof a dot is used only for a picture element having a high-contrastdensity.

(v) The standard diameter of a dot is used only for a picture elementhaving a dark density.

It is to be noted that the standard diameter of a dot can be obtained bythe standard driving current.

Second Embodiment

The above-described first embodiment has a configuration in which thesize of a printed dot in the LBP recording unit is controlled inaccordance with binary data and a flag indicating the density level ofan image.

A second embodiment which will be described below has a configuration inwhich a printer using a thermal head (a thermal-head printer, THP), suchas a printer for recording on heat-sensitive recording paper or aprinter using a heat-transfer method and the like, is used instead ofthe LBP in the first embodiment.

FIG. 11 is a block diagram showing the second embodiment. In FIG. 11,components indicated by like numerals as those in FIG. 1 have identicalconfigurations, and explanation thereof will be omitted.

A thermal-head printer 7 performs switching of the driving electricpower for heating resistors used as a thermal head according to datawhich have been binary coded by the error diffusion method in the readerunit 1, and thereby forms an image by either coloring heat-sensitivepaper or transferring ink of an ink ribbon to ordinary paper by turningon the power.

A receiver 8 includes a thermal-head printer unit.

It is to be noted that, in the second embodiment, the comparator 416 inFIG. 4, which indicates the detail of the binary-coding processing unit205 in the reader unit 1 (FIG. 2), is unnecessary.

FIG. 12 shows the detail of the thermal-head printer 7.

A thermal head control circuit 10 controls the thermal head by thecontrol of the CPU 104.

A shift register 11 converts serial data which are binary data sent fromthe thermal head control circuit 10 into parallel data. A latch circuit12 temporarily stores data converted into parallel data in the shiftregister 11. There are also shown a driver circuit 13, heating resistors14, a power supply circuit 15 for the thermal head for supplying adriving electric power to heating resistors 14, and a ceramic substrate16 which includes the heating resistors 14.

Now, the operation will be hereinafter explained.

The thermal head control circuit 10 transfers binary data which havebeen binary coded in the reader unit 1 to the shift register 11 asserial data signals.

The shift register 11 stores data for one line, and sends the data tothe latch circuit 12 as parallel data. The latched data for one line aresent to the driver circuit 13 by a latch signal from the thermal headcontrol circuit 10. The driver circuit 13 forms the AND of the binarydata from the latch circuit 12 and a printing-strobe-width signal (forexample, 0.4 msec) from the control circuit 10, and heats the heatingresistors 14 on the ceramic substrate 16.

By repeating the above-described operation for plural lines, it ispossible to record data which have been binary coded by the errordiffusion method using the thermal-head printer.

A recorded example by a thermal-head printer is shown in FIG. 13.

FIG. 13(a) shows a dark portion in which the number of black pictureelements 1 after binary-coding processing is large, and FIG. 13(b) is adiagram in which the binary data in FIG. 13(a) are recorded.

FIG. 13(c) shows a highlight portion in which the number of blackpicture elements 1 after binary-coding processing is small, and FIG.13(d) is a diagram in which the binary data in FIG. 13(c) are recorded.

As is apparent from FIG. 13, in the case of a thermal-head printer, thediameter of a printed dot becomes large in the case of a dark portion inwhich there are many black dots in surrounding picture elements, and thediameter of a printed dot becomes small in the case of a highlightportion in which there are few black dots. This is caused by thefollowing reason. Since the thermal-head printer performs printingoperation by temperature rise and cooling of the head, a printed dotbecomes large due to the heat-storage effect of the head when black dotsare continuously printed. On the contrary, since the head is cooled inthe case of few black dots (in the case of many whites), the temperatureof the head does not reach the heating temperature for the standardprinting even if energy is supplied to the head, and so the diameter ofa printed dot becomes small.

That is, in the first embodiment, the diameter of a printed dot ischanged by controlling the laser driving current of the laser-beamprinter, while in the case of using a thermal-head printer, it ispossible to change the diameter of a printed dot by utilizing theheat-storage effect of the head.

That is, since the diameter of a dot beomes large in a dark portion asshown in FIG. 13(b), it is possible to prevent the generation of whitespaces between dots.

Moreover, since the diameter of a dot becomes small in a highlightportion as shown in FIG. 13(d), it is possible to prevent a connectionof dots, and so prevent the generation of a unique striped pattern inthe error diffusion method.

As shown in FIG. 11, it is possible to transmit binary data which havebeen binary-coded by the error diffusion method to the receiver 8 viathe communication control unit 4. In this case, since it is notnecessary to transmit a flag indicating the density level of an image asin the first embodiment, it is possible to increase communicationefficiency.

When data are sent to an apparatus in communication, the CPU 104performs control in accordance with a transmission instruction from theoperating unit 6.

The receiver 8 is provided with a recording unit having the sameconfiguration as that of the thermal-head printer 7, and the recordingunit controls the thermal head according to binary data which have beensent to perform recording.

As explained above, according to the second embodiment, by means ofrecording binary data which have been binary coded by the errordiffusion method by a thermal-head printer, it is possible to controlthe size of the diameter of a dot in accordance with the denity of animage.

It is possible thereby to prevent the generation of a unique stripedpattern in the error diffusion method in highlight portions, and also toprevent the generation of white spaces between dots in dark portions,and thus to reproduce an excellent image.

Further, according to the second embodiment, as well as the firstembodiment, the binarizing process is based on the error diffusionmethod and the brightness data of a predetermined value or more is setto the density data 0, so that the increase amount of the errors in theerror diffusion method can be set to 0.

The particle-like noise in the highlight portion can be reliablyeliminated at a high level.

On the other hand, by eliminating the particle-like noise in thehighlight portion, the encoding process in the facsimile apparatus canbe also efficiently executed.

Although, in the present embodiment, a case in which image data aresubjected to a binary-coding processing has been explained, the presentinvention can also be used in a case in which image data are subjectedto a multiple-number-coding processing.

Furthermore, although, in the first embodiment, a case in which the sizeof the diameter of a dot is controlled by a laser-beam printer, it isalso possible to control the size of the diameter of a dot bycontrolling an ink-discharge amount in an ink jet printer.

As explained above, according to the second embodiment of the presentinvention, by outputting image data which have been quantized by theerror diffusion method, using a thermal-head printer, it is possible toprovide an image processing method and apparatus which can reproduce orcommunicate an excellent image having a high resolution with a simpleconfiguration.

Next, an embodiment when the binary-coding processing unit in FIG. 2 ispartially modified is shown in FIG. 14.

In FIG. 14, the comparator 416 shown in FIG. 4 is omitted.

An adder 901 adds error values to the density of an original pictureelement. A comparator 902 converts multivalued data into binary data bya threshold value. An error arithmetic circuit 903 calculates an errorgenerated in a picture element in question. An error-distribution-valuearithmetic circuit 904 calculates error values to be distributed tosurrounding picture elements according to a weighing matrix. There arealso shown latch circuits 905, 907, 910, 912 and 914, adders 906, 908,911 and 913, a memory 909 for storing error values, and a timinggeneration circuit 915 for providing a timing for a latch 907 and latchcircuits 905, 907, 910, 912 and 914.

The output data X_(i),j from the conversion table 204 in FIG. 2 and thedata E_(i),j from the latch 914 are added in the adder 901, and anoutput D_(i),j is obtained.

That is, D_(i),j =X_(i),j +E_(i),j, where E_(i),j is error data to beadded to the picture element (i,j) in question. The corrected densityD_(i),j of the picture element (i,j) in question is binary-coded by thecomparator 902, and is output as binary-coded data Y_(i),j.

    ______________________________________                                               D.sub.i,j ≧ T                                                                       ∴Y.sub.i,j = 63                                          D.sub.i,j < T                                                                              ∴Y.sub.i,j = 0.                                   ______________________________________                                    

The printer performs on (black)/off (white) of a dot according to thevalue of the binary-coded data Y_(i),j. That is, the printer prints awhite when the input signal is 0, and a black when the input signal is63.

The data D_(i),j of the picture element (i,j) in question aftercorrection are sent to the error arithmetic unit 903. The errorarithmetic unit 903 computes an error value which is generated when thepicture element (i,j) in question is binary-coded. That is, the errorE_(i),j can be expressed as follows;

    E.sub.i,j =D.sub.i,j -Y.sub.i,j.

The error E_(i),j is distributed to five picture elements surroundingthe picture element (i,j) in question, according to a weighting matrixshown in FIG. 15. In FIG. 15, (i+1,j) is a picture element to which asurplus of the error is allocated as described below.

In order to calculate these distribution amounts, the error E_(i),j issent to the error-distribution-value arithmetic circuit 904. An exampleof distribution of the value of the density X_(i),j of the pictureelement in question to surrounding five picture elements is shown inFIG. 16.

The operation performed in the error-distribution-value arithmetic unit904 will be explained illustrating a numerical example. If it is assumedthat the density X_(i),j of the original image of the picture element inquestion is (34), and the sum of errors diffused from surroundingpicture elements to the picture element in question is E_(i),j =-5, thedensity D_(i),j of the picture element after correction becomes D_(i),j=34+(-5)=29. If it is assumed that the threshold value is T=32, thedensity of the output image becomes Y_(i),j =0, and an error E_(i),jgenerated at (i,j) becomes ##EQU7##

The sum of weightings of the weighing matrix in FIG. 15 exclusive of(i+1,j) becomes 8 from ##EQU8##

In a digital operation, a division by the divisor 8 can be realized bytaking the upper 3 bits of the dividend as the quotient. The lower 3bits becomes a remainder surplus.

When distribution values for E_(i),j =29 are calculated according to theweighting matrix in FIG. 15, ##EQU9##

The surplus is the lower 3 bits of E_(i),j, that is, R_(i),j =5. In thepresent embodiment, R_(i),j is carried over to picture element (i+1,j).The weighting coefficient for (i+1,j) is 0.

The A_(i),j, B_(i),j, C_(i),j and R_(i),j enter into the adder 906, theadders 908 and 911, the latch 905 and the adder 913, respectively.

The memory 909 is used for storing error values for the (j+1)-th line.

Since the distribution processing of errors is nearly identical as thatin the case of FIG. 6, an explanation thereof will be omitted.

By repeating the above-described processing for plural lines, it ispossible to realize a binary-coding processing by the error diffusionmethod.

As described above, according to the second embodiment shown in FIG. 14,when realizing the error diffusion method by hardware, it is possible toprovide a circuit with a simple configuration without a deterioration inpicture quality, without using a decimal-point arithmetic circuitrywhich is large-scale hardware.

Moreover, by allocating a surplus to neighboring picture elements whichhave a high correlation with a picture element in question, it ispossible to prevent a decrease in resolution.

Although the surplus R_(i),j is allocated to picture element (i+1,j)adjacent to the picture element (i,j) in question in FIG. 15, a similareffect may also be obtained by allocating the surplus to picture element(i+2,j) as

    ______________________________________                                                 i - 1 i         i + 1   i + 2                                        ______________________________________                                        j                  x         1     □                               j + 1      1       4         2                                                ______________________________________                                    

Third Embodiment

A connection of dots is prevented in the first embodiment by aconfiguration in which the size of a printed dot in an LBP recordingunit is controlled in accordance with a flag indicating the densitylevel of an image, and in the second embodiment by a configuration inwhich a thermal-head printer is used instead of the LBP.

In a third embodiment which will be hereinafter explained, thegeneration of a striped pattern (a connection of dots) which causes adeterioration in picture quality in the error diffusion method isfurther reduced and so an excellent image is reproduced, by using athermal-head printer in a recording unit and controlling the size of aprinted dot.

The third embodiment of the present invention will be hereinafterexplained in detail with reference to the drawings.

FIG. 17 is a block diagram showing the third embodiment of the presentinvention. In FIG. 17, a reader unit 1001 comprises an image input unit1104 for reading an image of an original document and an imageprocessing unit 1105 for performing a quantization processing of inputimage data into binary data using the error diffusion method. A randomaccess memory (RAM) 1102 is used for image processing and as workingareas for a CPU 1101. The CPU 1101 controls the reader unit 1001 and athermal head recording unit 1002 which will be described later.

Control signals and image data in each block are transferred through asystem bus 1103. A read-only memory (ROM) 1106 stores control programswhich the CPU 1101 executes. A thermal head control circuit 1107controls the thermal head recording unit 1002 by the control of the CPU1101.

The thermal head recording unit 1002 performs image formation accordingto data which are binary coded by the error diffusion method.

A shift register 1108 converts serial data which are binary data sentfrom the thermal head control circuit 1107 into parallel data. A latchcircuit 1109 temporarily stores data converted into parallel data in theshift register 1108. There are also shown a driver circuit 1110, heatingresistors 1111, a power supply circuit 1112 for the thermal head forsupplying driving electric power to heating resistors 1111, and aceramic substrate 1113 which includes the heating resistors 1111.

In FIG. 17, the reading unit 1001 has a configuration which is entirelyidentical as that shown in FIG. 2, and an explanation of each componentwill be omitted.

FIG. 18 is a block diagram showing the detail of a binary-codingprocessing unit (205 in FIG. 2) of the reader unit 1001 in FIG. 17.

Since the configuration of FIG. 18 is identical as that of FIG. 4 exceptthat the comparator 416 in FIG. 4 is removed, like components areindicated by like numerals and an explanation thereof will be omitted. Aweighting matrix used in the error-distribution-value arithmetic circuit404 is also identical as that shown in FIG. 5.

It is possible to perform binary coding using the error diffusion methodby using a binary-coding circuit shown in FIG. 18.

The processing when image data which have been binary coded in the imageprocessing unit 1105 shown in FIG. 17 are recorded in the thermal headrecording unit 1002 shown in FIG. 17 will be hereinafter explained.

FIG. 19 is a flow chart showing a recording processing procedure. Theflow chart is stored in the ROM 1106, and the CPU 1101 executes the flowchart. First, the process proceeds to step S1, where a parameter t₁, forexample 0.3 msec, is set in a printing-strobe-width determinationcircuit of the thermal head control circuit 1107.

A strobe width represents a current-passing time for heating resistors1111. Strobe signals (1)--(N) in FIG. 17 are used for driving 1/N drivercircuits among driver circuits 1110 for one line, respectively. Forexample, when there are 2048-bit driver circuits for one line and strobesignals are (1)-(4), each strobe signal is used for driving drivercircuits for 512 bits.

At step S2, 2048-bit (one-line) binary data (DATA)₁ and data (DATA1)₁which are the logical product of random numbers of 2048 bits aretransferred to the shift register 1108. That is, (DATA1)₁ is representedby the following expression: ##EQU10## This operation is performed inthe thermal head control circuit 1107. A random number generationcircuit generates a pulse train each pulse of which is 0 or 1 and whichis synchronized with 1 bit of (DATA)₁.

At step S3, the data (DATA1)₁ in the shift register 1108 is latched in alatch circuit 1109.

At step S4-1, strobe signal (1) having a pulse width t₁ (0.3 msec inthis case) is switched on, and thereby each corresponding heatingresistor 1111 is heated once when the value of the data latched in thelatch 1109 is 1. Heat-sensitive paper is thereby colored. By repeatingthis operation up to strobe signal (N), a first recording for one lineis completed.

At step S5, a parameter t₂ (t₂ =0.4 msec) is then set in theprinting-strobe-width determination circuit of the thermal head controlcircuit 1107 in order to perform a second printing on the same line.

At step S6, the above-described binary data (DATA)₁ for one line areagain read from the RAM 1102 and transferred to the shift register 1108.The (DATA)₁ have been written in the RAM 1102 at the first recording.

At step S7, the data (DATA1)₁ in the shift register 1108 are latched inthe latch circuit 1109.

At step S8, strobe signal (1) having a pulse width t₂ (t₂ =0.4 msec) isswitched on, and each corresponding heating resistor 1111 is heatedonce. By repeating this operation up to strobe signal (N), the secondrecording processing for one line is completed. An example of anappearance of the printing in which one line has been printed twice isshown in FIG. 20.

Now, as shown in FIG. 20, a case in which 10-bit (DATA) 1201 are printedwill be investigated.

First, the logical product of a pulse train 1202 consisting of randomnumbers of 0 or 1 and the (DATA) 1201 is obtained to make it DATA1 1203.

When printing is then performed with a strobe width of t₁ =0.3 msec,black dots on paper are formed like dots 1204.

Since the energy provided by the strobe width t₁ is smaller than theenergy to be originally provided, the size of the dot becomes smallerthan the size of a proper dot.

Next, DATA 1201 are printed with a strobe width of t₂ =0.4 msec at aposition which is identical as that where (DATA1) 1203 have originallybeen printed. As a result, black dots on the paper are formed like dots1205.

That is, two dot sizes, i.e., black dots (DOT1) printed by both t₁ andt₂ and black dots (DOT2) printed only by t₂, randomly appear on thepaper. The relationship between the two sizes is as follows:

    (DOT1)>(DOT2).

The dot size becomes larger when printing is performed twice, becausethe heat from the first recording remains in the heating resistors.

By repeating the above-described processing for plural lines, it ispossible to form an image by changing the sizes of dots.

According to the present embodiment, it is possible to preventconnection of dots, since the dot size is switched between large andsmall. An appearance of this type of printing is shown in FIG. 21.

By thus preventing connection of dots, it is possible to prevent thegeneration of a unique striped pattern in the error diffusion method.However, in the above-described embodiment, although it is possible toprevent the generation of a striped pattern in highlight or half-toneportions of an image, there occur white spaces due to the fact that dotsbecome small in dark portions of an image.

Hence, in the following embodiment, a case in which the size of a dot isswitched in accordance with the density of an image will be explained.

In this case, it is possible to perform the binary-coding processingperformed in the image processing unit 1105 in FIG. 17 in thebinary-coding processing unit shown in FIG. 4.

In FIG. 4, a comparator 416 determines to which among highlight signal,dark signal and half-tone signal the input image signal X_(i),j belongs,and outputs a flag for each signal.

That is, the X_(i),j is compared with two threshold values TD1 and TD2(TD1<TD2)

    ______________________________________                                        X.sub.i,j ≦ TD1                                                                        ∴Flag = 0 (highlight signal)                          TD1 > X.sub.i,j > TD2                                                                         ∴Flag = 1 (half-tone signal)                          X.sub.i,j ≧ TD2                                                                        ∴Flag = 2 (dark signal),                              ______________________________________                                    

and outputs a flag corresponding to each gradation level.

FIG. 22 is a flow chart showing a procedure for recording data whichhave been binary-coded in the binary-coding processing unit shown inFIG. 4. The process first proceeds to step S11, where a parameter t₁,for example, t₁ =0.3 msec, is set in the printing-strobe-widthdetermination circuit of the thermal head control circuit 1107.

At step S12, binary data DATA (1) to be first printed are set in theshift register 1108. The DATA (1) are prepared in the thermal headcontrol circuit 1107. The flow chart for preparing the DATA (1) is shownin FIG. 23. At step S20, if the value of a binary-coded output signalY_(i),j is Y_(i),j =1 (printing of black), the process proceeds to stepS21. If Y_(i),j =0 (printing of white), the process proceeds to stepS24, where it is arranged that Y_(i),j =0.

At step S21, the value of Flag 417 is investigated, and if Flag=2 (darksigal), the process proceeds to step S25, where it is arranged thatY_(i),j =1, and if Flag≠2, the process proceeds to step S22.

At step S22, the value of Flag 417 is investigated, and if Flag=1(half-tone), the logical product of an output signal of the randomnumber generation circuit for generating a signal of 0 or 1 and theY_(i),j is obtained to make it Y_(i),j.

If Flag≠1, an arrangement is made that Y_(i),j =0.

A pulse train of 2048 Y_(i),j 's obtained in the above-describedoperation is DATA (1).

At step S13 in FIG. 22, the data DATA (1) in the shift register 1108 arelatched in the latch circuit 1109.

Strobe signal (1) having a pulse width of t₁ is switched on at step S14,and thereby the corresponding heating resistors 1111 are heated oncewhen the value of data latched in the latch 1109 is 1.

Heat-sensitive recording paper is thereby colored.

By repeating this operation up to strobe signal (N), a first recordingprocessing for one line is completed.

The process further proceeds to step S16, where a parameter t₂ (t₂ =0.4msec) is then set in the printing-strobe-width determination circuit ofthe thermal head control circuit 107.

At step S16, data DATA (1) to be printed again are set on the sameposition where the DATA (1) have been printed.

At step S17, the data DATA (1) in the shift register 1108 are latched inthe latch circuit 1109.

At step S18, a strobe signal having a pulse width of t₂ is switched on,and the corresponding each heating resistor 1111 is heated once when thevalue of data latched in the latch 1109 is 1.

The appearance resulting when one line is separately printed twice isshown in FIG. 24.

FIG. 24 shows a case in which 10-bit DATA (1) 1301 are printed. It isassumed that 0 represents a white, and 1 represents a black.

According to the above-described flow chart in FIG. 23, the DATA (1)becomes DATA (1) 1304.

When the DATA (1) 1304 are printed with a strobe width t₁, dots onheat-sensitive paper becomes dots 1305. Since the energy provided to thepaper with the strobe width t₁ is smaller than the energy to beoriginally provided (an energy by which a proper dot size by the thermalhead is obtained), the dot size is smaller than a standard size.

DATA (1) 1301 are then printed with a strobe width of t₂ on the sameposition where the DATA (1) 1304 have been printed.

Finally, dots on the heat-sensitive paper become dots 1306.

Two kinds of dot sizes consisting of black dots (Dot 1) printed by thetwo strobe width t₁ and t₂ and black dots (Dot 2) printed only by the t₁appear on the heat-sensitive paper.

That is,

Dot 2, when the input image signal X_(i),j is from a highlight portion,

Dot 1, when the input image signal X_(i),j is from a dark portion, and

Dot 1 or Dot 2, when the input image signal X_(i),j is from a half-toneportion. Either of the above-described three cases is selected andprinted.

The relationship between the sizes of dots is

    (Dot 1)>(Dot 2).

As explained above, according to the third embodiment, connection ofdots is prevented, since printing is performed reducing the dot size inhighlight portions of an image. That is, it is possible thereby toprevent the generation of a unique striped pattern in the errordiffusion method.

Furthermore, since printing is performed with random dot sizes, largeand small, in half-tone portions, it is possible to prevent connectionof dots and provide gradation in half-tone portions.

Moreover, since printing is performed with a large dot size in darkportions of an image, it is possible to prevent the occurrence of whitespaces between dots.

Thus, according to the third embodiment, by changing the size of aprinted dot in accordance with the density of an input image, it ispossible to prevent the generation of a unique striped pattern in theerror diffusion method in highlight and half-tone portions of an image,and also to prevent the occurrence of white spaces in dark portions ofan image.

Although the dot size is changed by changing the strobe width in twoways in the present embodiment, the dot size may also be changed bycontrolling voltage or current.

Furthermore, although, in the present embodiment, a case is discussed inwhich image data are subjected to binary-coding processing by the errordiffusion method as a method of quantization, the present invention mayalso similarly be utilized for a multiple-value-coding processing.

In the case of a color image, the present invention may be executed byproviding the circuit in the present embodiment for three colors, R, Gand B.

While the present invention has been explained in reference to thepreferred embodiments, it is not limited to the above-describedembodiments, but various changes and modifications are possible withinthe scope of the appended claims.

What is claimed is:
 1. An image processing apparatus comprising:readingmeans for reading an image of an original and generating brightnessdata; converting means for converting the brightness data generated bysaid reading means into density data; binarizing means for binarizingthe density data obtained by said converting means, by an errordiffusion method; and outputting means for outputting binary dataobtained by said binarizing means to a laser beam printer, wherein saidconverting means converts to 0 density data for brightness data of apredetermined value or more.
 2. An image processing apparatus accordingto claim 1, wherein said binarizing means binarizes the density data bycorrecting an error between the density data converted by saidconverting means and the binary data after binarization.
 3. An imageprocessing apparatus according to claim 1, further comprisingdetermination means for determining to which among highlight, half-toneand dark regions the density level of the density data belongs.
 4. Animage processing apparatus according to claim 3, wherein said outputtingmeans changes a size of a dot in accordance with a result ofdetermination performed by said determination means.
 5. An imageprocessing apparatus according to claim 4, wherein, when saiddetermination means determines that the density level of the densitydata belongs to a highlight region, said outputting means outputs thesize of the dot smaller than that when the density level is determinedto belong to a dark region.
 6. An image processing apparatus accordingto claim 4, wherein said outputting means randomly changes the size ofthe dot between large and small when said determination means determinesthat the density level of the density data belongs to a half-toneregion.
 7. An image processing apparatus according to claim 4, whereinsaid outputting means changes the size of the dot by changing theintensity of a laser beam in accordance with the determination resultobtained by said determination means.